In an electrochromic device, a physical/chemical change is produced in response to an induced electric field. The result is a change in the reflective (or transmissive) properties of the device with respect to electromagnetic radiations, e.g., UV, visible and IR radiations. Such devices, one embodiment being shown as item 10 in FIG. 1, generally comprise a film of electrochromic material 12 and an ion-conductive insulating layer 14 which functions as an electrolyte layer. The film and the electrolyte layer are in surface contact with each other for exchange of ions between the electrochromic film and the electrolyte layer. Two conductive electrode layers, 16 and 18 in FIG. 1, at least one of them being transparent, are disposed on the opposite outer surfaces of the film and the electrolyte layer to provide means for applying a voltage across the combined thickness of the electrochromic film and the electrolyte layer. As shown in FIG. 1, electrode layers are provided on substrates 20 and 22, which substrates may be of a material such as glass. Depending on the ion providing and ion storage capacity of ion conductive layer 16, a counter electrode located between ion conductive layer 14 and electrode layer 18 may be used. The electrodes are provided with external electrical leads 24 and 26 connected to a voltage providing source 28. Application of a voltage of proper polarity across the electrodes causes coloration of the electrochromic layer. By reversing the polarity of the applied voltage, the colored electrochromic layer will be uncolored (bleached). Changing from the bleached state to the colored state or from the colored state to the bleached state is termed "switching". The electrochromic material may be "persistent" in its colored state which means that it has the ability to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "open circuit memory" or simply "memory". Electrochromic devices of this type have been described for several uses, such as for image display, for light filtering, etc. See, e.g., U.S. Pat. Nos. 3,708,220, 4,194,812; 4,278,329; 4,645,308; 4,436,769; 4,500,878; 4,150,879; 4,652,090; 4,505,021; and 4,664,934.
In such devices, the electrochromic film usually comprises an inorganic metal oxide material, most commonly a transition metal oxide, in particular: tungsten oxide. When tungsten oxide is the electrochromic material, the electrolyte layer is adapted to provide a positively charged light cation, preferably, a proton or a lithium ion. The electrolyte layer may be a liquid electrolyte solution like lithium perchlorate in propylene carbonate or a gel electrolyte like polyvinyl butyral-methanol doped with LiCl. The electrolyte layer may also be a solid electrolyte which comprises polymers or copolymers containing acidic groups such as polystyrene sulfonic acid, propylene oxide or polyethylene oxide.
It would be desirable, however, to have an electrochromic device which comprises less layers and is hence less complex to fabricate. Additionally, it would be desirable if the device, in addition to being able to reduce transmission of visible light, would also be able to substantially reduce transmission of IR wavelength radiation, i.e., keep radiation of the type which generates heat from passing through the device. This would be particularly useful if the device is used as a window of a building or automobile.